1. Field of the Invention
The present invention relates to a time base corrector having dropout compensating function, for use in an apparatus for reproducing a composite video signal recorded on a recording medium.
2. Description of Background Information
When a video signal recorded on a video disc is simply demodulated and reproduced, a reproduced video signal may include fine jitter due to eccentric components of the video disc and vibrations caused in the mechanical system of the apparatus, resulting in a time base error which represents a deviation from a reference time base of the reproduced video signal. For correcting this time base error, a digital time base corrector is known which converts a reproduced video signal to digital data by an A/D convertor, writes the digital data once into a memory, reads out them in the writing order in synchronism with a reference timing signal, and reconverts the read digital data to an analog signal by a D/A convertor.
A conventional time base corrector is disclosed, for example, in Japanese Patent Application Kokai No. 1-93273. In this time base corrector, at least one of a horizontal synchronizing signal and a color burst signal is separated and extracted from a reproduced video signal. A phase locked loop (PLL) circuit is provided for receiving the separated signal. The PLL circuit generates a clock signal, the phase of which is synchronized with a time base error included in the reproduced video signal. The clock signal is phase modulated by a phase modulator means in accordance with a phase comparison-output of a phase comparator means arranged in the PLL circuit. The clock signal, after the phase modulation, serves as a sampling timing signal for the A/D convertor. In this way, the clock signal follows even high frequency components of the time base error, thus making a correction of such time base error.
Some of such time base correctors include a dropout compensating function. For example, as disclosed in Japanese Utility Model Registration Application Kokai No. 3-125577, a dropout detector circuit is provided for detecting dropout in a reproduced video signal, such that a memory write operation is inhibited when dropout is detected, and digital video data which have already been stored in the memory are read out during the period in which the writing operation is inhibited.
In such a conventional digital time base corrector including a dropout compensating function, assume that the memory is provided only with a capacity of storing video signals for 1 H (horizontal scan period). In this case, however, since the phase of a chrominance signal is inverted every 1 H period in the NTSC video signal, the memory must have a capacity of storing image data for at least 2 H in order to maintain the interleave relationship between a luminance signal and a chrominance signal of the video signal. It is nevertheless desirable to reduce the memory capacity for a lower cost.